Thursday, October 28, 2010

Antarctic Glaciers Surged After 1995 Ice-Shelf Collapse

John Roachfor National Geographic News

March 6, 2003

When a huge floating shelf of ice hinged to the northern end of the
Antarctic Peninsula disintegrated in January 1995, several glaciers that
were backed up into it surged towards the sea, according to a pair of
Argentinean researchers.

The discovery marks the first positive evidence that glacial surge follows an ice shelf collapse. It may lead scientists to revive the previously discarded theory that ice shelves acts as dams that prevent inland glaciers from slipping into the seas.

"We think that our discovery strongly supports the view of the ice shelves as major dams for the inland ice," said Hernán De Angelis, a scientist with the Glaciology Division of the Instituto Antártico Argentino in Buenos Aires.

De Angelis and colleague Pedro Skvarca made their discovery through analysis of airborne mapping data, satellite imagery, and flights over the area of the peninsula affected by the collapse of the northern section of the Larsen Ice Shelf. They report their findings in the March 7 issue of the journal Science.

For more than 30 years, scientists have debated whether or not ice shelves act as barriers for inland ice. The speed at which glaciers move, a rate technically known as glacial flow, is largely determined by the type of surface glaciers rest on, according to recent computer modeling studies. Those studies lead many scientists to discount the theory that ice shelves act as a sort of brake.

The debate is important, say scientists, because if the same disintegration scenario were to play out with major ice shelves such as the Ross and Filchner-Ronne, the entire West Antarctic Ice Sheet could collapse and cause global sea levels to rise upwards of 16 feet (5 meters)provided ice shelves were the only thing holding the glaciers back from the sea.

"In certain circumstances ice shelves probably do act as a barrier. When removed the glaciers will undergo some change, as described in the paper," said Chris Doake, a scientist with the British Antarctic Survey in Cambridge, England. "But you cannot extrapolate this behavior to the rather different physical conditions seen around much of the West Antarctic Ice Sheet."

Glacial Surge

De Angelis and Skvarca found that five of the six major streams of ice that fed into the northern section of the Larsen Ice Shelf dramatically surged towards the sea and then retreated in the years following the 1995 disintegration of the shelf.

Their evidence for this surging includes several locations where blocks of ice sit stranded about 66 to 131 feet (20 to 40 meters) above glacier surfaces. These so-called ice terraces are formed when a glacier suddenly lowers as a result of a surge.

In addition, maps of the glaciers made in March 2002 show that they have retreated since their surge, an indication that the glaciers dumped much of their ice into the seas, said De Angelis.

"Our discovery is indicating that a strong coupling between ice-stream and ice-shelves exists," he said.